The present invention relates to a control system for four-wheel drive vehicles with a continuously variable belt-drive transmission, and more particularly to a system for controlling a transfer device for four-wheel drive vehicles.
A four-wheel drive vehicle of the type in which a transfer clutch is provided for transmitting the output torque of the transmission to auxiliary drive wheels is known. In such a vehicle, a power transmission train can be converted from two-wheel drive to four-wheel drive by engaging the transfer clutch. The continuously variable transmission (CVT) is provided with an oil supply source for controlling the transmission ratio, and hence a hydraulic transfer clutch which is operated by pressurized oil supplied from the oil supply source is provided for the four-wheel drive transmission system.
Japanese Patent Application Laid-Open 61-249834 discloses a four-wheel drive vehicle in which a transfer clutch in the form of a fluid operated multiple-disk friction clutch is employed. The line pressure for controlling the transmission is controlled in dependency on driving conditions, and the pressure is applied to the transfer clutch for controlling the transmitted torque for the rear drive wheels in the four-wheel drive mode.
However, the line pressure is supplied to servo devices of the transmission for preventing a drive belt of the transmission from slipping on the pulley of the transmission. When the line pressure is applied to the transfer clutch, the amount of oil is reduced, resulting in a decrease of the line pressure in a moment. The reduction of the line pressure causes the belt to slip. Therefore, it is necessary to provide a device for preventing the line pressure from decreasing at the engagement of the transfer clutch.
Further, there is a problem that when the transfer clutch is engaged in heavy load driving, an extremely large torque is transmitted in the auxiliary power transmission system, which causes a shock in the system and belt slipping.